1. Field of the Invention
The invention relates to an image display device comprising an input for receiving an image update signal; an image memory for storing image information; a processing circuit coupled to the input and the image memory, for effecting a modification of a part of the image information stored in the image memory and affected by the image update signal; and a display unit coupled to the image memory, for displaying an image according to the image information.
2. Description of the Related Art
Such an image display device is used, for example, with a personal computer (PC). The PC contains a CPU and a graphics card, for example a so-called VGA card, coupled to a display unit. In operation, the CPU sends image update signals to the graphics card, for example, a signal to update the color value of a pixel, or to draw a line with a certain color, or to update the color values in a rectangular window. The graphics card contains an image memory in which information representing an image is stored, usually with individual memory locations containing pixel values for individual pixels. Each time the graphics card receives an update signal, it processes this update signal and modifies the information in the image memory according to the update signal, for example, by replacing a single pixel value, or replacing pixel values along a line or in a rectangular window. The graphics card periodically outputs an image signal derived from the image information from the image memory to the display unit.
The display unit periodically receives the image signal and converts the image signal into control signals with which the light output of individual pixels is controlled. Corrections are applied which depend on the physical properties of display unit, changes in the settings of the control organs of the display unit, etc. Such corrections are transparent to the CPU which produces updates that are valid for any type of display unit with any setting. The corrections range from simple corrections, for example, to account for the brightness, contrast and saturation setting of the display unit, to more complicated corrections, for example, to correct for beam repulsion effects, convergence errors and cathode drive modulation effects in a cathode ray tube or to suppress Moirxc3xa9effects caused by the display raster.
It would be desirable if it were possible to convert the image signal into control signals performing many of these corrections digitally. However, performing too many corrections digitally would require an extreme amount of processing power, especially for high resolution display units.
It is an object of the invention to provide and image display apparatus in which the need for processing power to apply display unit dependent corrections to the image information is reduced.
According to the invention, the image display apparatus is characterized in that the processing circuit is arranged to generate the modification dependent on a display unit dependent correction. Effectively, this places the graphics card logically together with the display unit in a single monitor, allowing the monitor to apply the display unit dependent corrections to the update signals received from the CPU before the image information in the memory is modified rather than after modification when the image information is outputted from the image memory to the display unit. Of course, some corrections that require little processing time may still be applied to the image signal output from the image memory to the display unit.
An update signal affects only part of the image and the amount of information in the update signals per unit time is usually much lower than that in the image signal output from the image memory per unit time. This means that a relatively low speed processing circuit (preferably a digital signal processor) usually suffices to apply the display unit dependent corrections.
An embodiment of the image display apparatus according to the invention is characterized in that the processing circuit is arranged to generate, normally, the modification dependent on the display unit dependent correction and on a further display unit dependent correction, the processing circuit being arranged to detect whether or not there is sufficient time to generate the modification dependent on both display unit dependent corrections, and if not to effect the modification of the image information in the image memory without accounting for the further display unit dependent correction. In this embodiment, the processing circuit is able to apply at least two corrections when it modifies the image information, for example, a correction to account for brightness setting and a correction to suppress Moirxc3xa9effect. When the processing circuit has sufficient time, both corrections are applied before the modification of the image information is effected. However, when there is not enough time, for example, because a large number of update signals has to be processed, one of the corrections is not applied, preferably, the correction of the effect that is generally least visible, or least visible in dynamic images.
A further embodiment of the image display apparatus according to the invention is characterized in that the image display apparatus comprises a correction table memory for storing information about whether or not the modification has been generated dependent on the further display unit dependent correction, the processing circuit being arranged to effect a further modification of the image information in the image memory responsive to the image update signal and generated dependent on the further display unit dependent correction, when the processing circuit has sufficient time to generate the further correction. In this way, the modification that has been effected in the image memory without accounting for a correction is corrected later when there is sufficient time.
An embodiment of the image display apparatus according to the invention is characterized in that the image display apparatus comprises a look-up memory for storing a set of corrected color values corresponding to respective input color values from a subset of colors that the display unit is capable of displaying, the processing circuit being arranged to generate the modification by reference to the color look-up memory in case the modification involves an input value for which a corrected color value is stored, and by computation otherwise. In this way, renewed computation of corrected modifications is avoided, reducing the required amount of processing.
A further embodiment of the image display apparatus according to the invention is characterized in that the image display apparatus comprises means for analyzing the image information so as to determine a set of most frequent input color values, said means updating the color look-up memory so that the color look-up memory contains corrected color values for said most frequent input color values. Thus, the apparatus is capable of optimizing its speed adaptively dependent on the statistics of the image. Usually the selected most frequent input colors will correspond to individual colors used for various interface item in a color scheme selected by the operating system running on the CPU. When the user switches to another color scheme, the image display apparatus will thus adapt itself to that other color scheme.
An embodiment of the image display apparatus according to the invention is characterized in that the image display apparatus comprises a further image memory for storing further image information, and means for effecting a further modification of a part of the further image information stored in the image memory and affected by the image update signal, the further modification being generated without accounting for said display unit dependent correction, the processing circuit generating the modification dependent on the update signal, the further image information and the display unit dependent correction. Thus, two image memories are used, one for uncorrected image information and one for corrected image information. The image memory for uncorrected image information is used for corrections where uncorrected pixel values are needed, such as, operations upon a pixel that need information about adjacent pixels, e.g., n-tap filter operations for interpolation, correction of convergence errors, Moirxc3xa9 suppression, bit block transfer functions.
The invention is especially advantageous when the display unit uses a CRT (cathode ray tube), because a CRT requires a considerable number of corrections for high quality image display.